Apparatus and methods for separating, detecting, and measuring trace gases

ABSTRACT

Apparatus and methods for sorting and detecting trace gases which undergo ion-molecule reactions. Reactant ions and product ions may be sorted in accordance with their velocity in an electric field at different regions of a drift cell, and multiple ion-molecule reaction regions may be provided. Different regions of the cell may be supplied with different gaseous media.

United States Patent Carroll 1 June 6,1972

[54] APPARATUS AND METHODS FOR SEPARATING, DETECTING, AND MEASURING TRACE GASES [72] Inventor: David I. Carroll, Lantana, Fla.

[73] Assignee: Franklin GNO Corporation, West Palm Beach, Fla.

[22] Filed: Jan. 9, 1969 [21] App1.No.: 790,108

52 us. 01 ..2s0/41.9 TF, 250/419 0, 250/419 SE 51 1111. c1. ..;...n01 39/36 58 Field of Search ..2so/41.9 TF, 41.90, 41.9 R,

[56] References Cited UNITED STATES PATENTS 7/1960 Parsons et al..... ..2s0/41.9

2,959,676 11/1960 Burk ..250/41.9 3,254,209 5/1966 Fite et al. ..250/4L9 SB Primary Examiner-Anthony L. Birch Attorney-Raphael Semmes ABSTRACT 28 Claims, 1 Drawing Figure ELECTROMETER SYNC DELAYED DELAYED DELAYED PULSE PULSE PULSE PULSE 1 APPARATUS AND METHODS FOR SEPARATING, DETECTING, AND MEASURING TRACE GASES BACKGROUND OF THE INVENTION This invention relates to apparatus and methods of ion classification and more particularly is concerned with improving the resolution of measurements perfonned upon trace gases which undergo ion-molecule reactions.

The copending application Ser. No. 777,964 of Martin J. Cohen, David I. Carroll, Roger F. Wemlund, and Wallace D. Kilpatrick filed Oct. 23, 1968 and entitled Apparatus and Methods for Separating, Concentrating, Detecting and Measuring Trace Gases, discloses Plasma Chromatography systems involving the formation of primary or reactant ions 7 and reaction of the primary ions with molecules of trace substances to form secondary or products ions, which may be concentrated, separated, detected, and measured by virtue of the difference of velocity or mobility of the ions in an'electric field. The primary ions may be produced by subjecting the molecules of a suitable host gas, such as air, to ionizing radiation, for example. The primary ions are then subjected to an electric drift field, causing them to migrate in a predetermined direction through a reaction space into which the sample or trace gas is introduced. The resultant collisions between primary ions and the trace gas molecules produce secondary ions of the trace gas in much greater numbers than can be produced by mere electron attachment, for example, to the trace gas molecules. The secondary ions are also subjected to the electric drift field and may be sorted in accordance with their velocity or mobility. The pressure within the Plasma Chromatograph drift cell is maintained high enough (preferably atmospheric) to ensure that the length of the mean free path of the ions in the cell is very much smaller than the dimensions of the cell. A specific system of the said copending application employs a pair of successively arranged ion shutter grids or gates for segregating the ion species in accordance with their drift time. The opening of the first gate is timed to pass a group of ions, which may comprise unreacted primary ions as well as secondary ions, and the opening of the second gate is timed to pass a portion of the group to an ion detection means. A two-gate drift cell has only a single velocity analysis. region. No provision is made for sorting of primary ions prior to the desired ion-molecule reactions, or for further sorting of secondary ions, and the ion-molecule reactions occur principally in the same region as primary ion formation.

BRIEF DESCRIPTION OF THE INVENTION It is accordingly a principal object of the present invention to provide improved apparatus and methods for separating and detecting trace gases with higher resolution than has heretofore been possible in the two-gate drift cell and with greater flexibility of operation. 7

Briefly stated, preferred embodiments of the apparatus and methods of the invention are concerned with Plasma Chromatography" systems which involve the formation of positive or negative ions by reactions between the molecules of trace substances and primary ions. The secondary ions are separated and detected in a drift cell by utilizing the difference in velocity or drift time of ions of different mass in an electric field. In accordance with one embodiment of the invention well defined sequential regions may be provided for primary ion formation, primary ion velocity sorting, primary ion-trace molecule reactions, secondary (trace) ion velocity sorting, and ion detection. Different gaseous media may be provided at the various regions for greater resolution and greater flexibility of operation.

BRIEF DESCRIPTION OF THE DRAWING The invention will be further described in conjunction with the accompanying drawing, which illustrates a preferred and exemplary embodiment, and the single FIGURE of which is a combined schematic and block diagram of a trace gas detector system of the invention.

2 DETAILED DESCRIPTION OF THE INVENTION Referring to the drawings, a Plasma Chromatography cell 10 in accordance with the invention may comprise a gas-tight envelope 12 enclosing a series of electrodes, which may be of plane parallel geometry, for example. Principal electrodes C and A may be arranged adjacent to opposite ends of the envelope, which may be a circular cylinder. When the apparatus is used to detect negative ions, as will be assumed for example, electrode C will be a cathode and electrode A an anode. When the apparatus is used to detect positive ions, the polarities will be reversed. The Plasma Chromatography cell described in the said copending application includes a pair of shutter grids or ion gates, such as the grids G1 and G2. In the apparatus of the present invention, however, at least one additional shutter grid or ion gate is provided. Preferably two additional ion gates or grids G3 and G4 are provided. Each of the shutter grids comprises two substantially coplanar sets of interdigitated parallel wires, alternate wires of each grid being connected together to form the two sets. Cathode C or the region of the envelope near this electrode is provided with an ionizing source, which will later be described in more detail. Anode A may be a collector plate and constitutes an output electrode.

Grids G1, G2, G3 and G4 are arranged in spaced sequence between the electrodes C and A and define therewith a plurality of sequential regions within the envelope 12. The electrodes may be spaced apart distances of the order of a few centimeters or less and the total cathode-to-anode spacing may be of the order of 10 cms. Inlet tubes 14, 16, 18 and 20 pemrit the introduction of gas into corresponding regions, namely, the region between C and G1, the region between G1 and G2, the region between G2 and G3, and the region between G3 and G4. Outlet tubes 22, 24, 26 and 28 are provided for exhausting gas from these regions. The region between G4 and A will contain gas introduced at inlet 20. By maintaining the gas flow transverse to the drift direction and by controlling the relative rates of gas flow it is possible to maintain the gaseous media in the various regions. In the form shown the gas flow is essentially transverse to the direction between the principal electrodes C and A. ElectrodeA may be connected to an electrometer 30, such as a Cary Instruments Model 401 (vibratingreed) type with current sensitivity of 10 arnperes at a time constant of 300 milliseconds.

An electric drift field is provided between the principal electrodes C and A. In the form shown the source of the drift field is a chain of batteries 32, the negative end of the chain being connected to the cathode C and the positive end to ground. Anode A is connected to ground through the input circuit of the electrometer 30. Alternatively, a resistor chain voltage divider may be employed in conjunction with a battery connected across the chain.

Adjacent elements of each shutter grid are normally maintained at equal and opposite potentials relative to a grid average potential established by the battery chain 32. Under these conditions the shutter or gate is closed to the passage of electrically charged particles. Potential sources which provide the equal and opposite potentials just referred to may be considered to be part of grid drive circuits within blocks 34, 36, 38, and 40, block 34 being entitled Sync Pulser" and the remaining blocks Delayed Pulse. As described in the said copending application, the components of these blocks are effective, at predetermined times, to drive the adjacent elements of the associated shutter grids to the same potential, the grid average potential, alternate grid wires being connected to the battery chain by resistors 42 and 44 to establish the grid average potentials. Taps on the chain of batteries are connected to a series of guard rings 46 spaced along the length of the envelope 12, which maintain the uniformity of the drift field.

Before a description of the operation of the invention is given, certain general observations will be helpful. The invention is capable of analyzing samples supplied from different types of sources. For example, the invention may be employed to detect chemicals in the atmosphere. In such applications as a gas chromatograph detector, in which event the sample is pure and relatively small to 10 grams), the sample time relatively short (fraction of a second), and the flow rate small 10 to 100 milliliters per minute A relatively small detection chamber 12 is preferred. The invention is also applicable to samples from general chemical processes. In such applications any of the foregoing characteristics may apply.

The invention employs an ion generator or ionizer, which is at or adjacent to electrode C. The ionizer may be a radioactive source such as a tritium or nickel 63 source, a corona source produced by a high potential between fine wires, a flame ionization source using the hydrogen, hydrocarbon or other flame, aphotoemissive source such as ultraviolet light, a surface efiect source such as a source of alkali ions produced by the impact of halogens on heated metallic surfaces (usually with precious metal content), a metastable molecule source such as argon, mercury, or organic radical which in interaction with normal molecules will ionize them, or a photoioniza tion source such as direct laser ionization of gas.

The invention employs a reactant or primary ion generation region, which may be a specially delineated region or a region combined with the secondary ion generation region. Three options are possiblei (l) the sample gas can be injected into the reactant ion generation region with a special added reactant gas, or (2) without an additional reactant gas, or (3) the reactant gas only can be injected and the sample gas inserted at a subsequent region In the apparatus illustrated the region between C and G1 is the reactant or primary ion generation region. The reactant gas may be a portion of the gaseous sample introduced to this region through pipe 14, such as oxygen in the ambient atmosphere, or may be introduced alone to this region, as from a separate source of oxygen. The trace gas to be detected may be a portion of the gaseous sample containing the reactant gas, or may be separately introduced at this region, or may be introduced at another region to be described hereinafter. The reactant ion generation region may produce a specific ion at its terminus merely by selection of the reactant or by permitting a particular ion-molecule reaction to reach completion before gate G1 is opened.

The invention may employ a reactant ion velocity sorter region, between grids G1 and G2. This permits the selection, by ion velocity analysis, of a particular reactant ion species for later reaction with a particular trace molecule species. A special non-reactant or inert gas, such as nitrogen, may be in-.

troduced by inlet 16 to the region between G1 and G2 for velocity sorting as set forth in the copending companion appli cation of David 1. Carroll, Martin J. Cohen, and Roger F. Wernlund, filed Dec. 3, I968 and entitled Apparatus and Methods for Separating, Detecting, and ,Measuring Trace Gaseswith Enhanced Resolution. Such a gasquenches ionmolecule reactions.

A specifically delineated region may be provided for reactant ion-trace gas molecule reactions, namely the region between grids G2 and G3. The sample containing the trace gas or gases to be analyzed is introduced through inlet 18. In a simpler form of the apparatus the sample gas may be introduced'at the region between C and G1 as mentioned previously.

The invention employs a sample ion velocity sorter region, between G3 and G4. This region may'also be provided with a non-reactive drift gas which is inert to the ions present so that ion-molecule reactions are quenched as set forth in connection with region Gl-G2. If an inert gas is used at the velocity sorting regions, the ions drift through these regions without spurious ion-molecule reactions which might affect apparentmobility.

The invention also employs a measurement or detection region, between G4 and A, where the ions which pass grid G4 impinge upon anode A and produce an output current which is measured by electrometer A typical mode of operation will now be described, and then comments will be made as to modifications.

A suitable gas such as dry air free of trace substances is fed to the region C-Gl by a suitable source of gas pressure, .such

as a pump in the gas flow path 14, 22. In this region primary ions of a reactant gas, such as oxygen in the air, are formed under the influence of the ionizing source. If a tritium foil cathode is employed, 'for example, negative oxygen primary ions will be formed very closely to the cathode, and thus at a localized region of the drift field. The primary ions drift in the direction of the anode A. At a predetermined instant a sync pulse is applied from source 34 to thegrid G1, momentarily to drive all of the wires of grid G1 to the grid average potential and thus to permit primary ions to enter the region Gl-G2.

In the region Gl-G2, assuming an inert gas supplied from a source connected to inlet 16, there will be no ion-molecule reactions, and the primary ions will merely drift toward grid G2, becoming grouped in accordance with their. velocity (mass). At a predetermined time delayed relative to the application of the sync pulse to grid G1 a pulse will be applied to grid G2 from source 36, momentarily to open grid G2 and to permit a particular reactant ionspecies to enter thespace G2-G3. A sample including trace gas to be detected, such as air containing Ethion, is inserted into this region from a source connected to pipe 18. In drifting through this region the primary ions encounter sample gas molecules. Although a majority of these collisions may be with non-reactive molecules, a small fraction of the collisions will be with the reactive trace molecules of interest. In these cases the primary ions will interact with the trace molecules to form secondary ions, which will have, in general, an appreciable difference in mobility from the primary ions.

' The ion flux at grid G3 will consist of unreacted primary ions and possibly several species of secondary ions. A sample of this mixed ion population is periodically admitted to the drift region between G3 and G4 when a pulse is momentarily applied to grid G3 from source 38 at a predetermined instant delayed with respect to the application of the pulse to grid G2 from source 36. The ions that pass through grid G3 enter the secondary ion velocity analysis region between grids G3.and G4, which, as stated above, may be provided with an inert gas from a source connected to inlet 20 so as to quench any ion ing of grid G3 a pulse is momentarily applied to grid G4, so i that a particular species of ions is passed to the detection region between G4 and A. The resultant output current in the anode circuit is integrated over several cycles to give a measurable current. By scanning the time of opening of grid G4 relative to grid G3 a drift time spectrum of the ion population in the region G3-G4 can be obtained .in the output and recorded to produce an output curve (current v. drift time). This permits the various secondary ion species to be separated and identified. Similarly, by scanning the time of opening of grid G2 relative to grid G1 particular species of primary ions may be passed to the ion-molecule reaction region between G2 and G3 and the resultant effect upon the output curve noted. It is thus possible to select particular species of primary ions for reaction with particular species of trace gas molecules reaction region between G2 and G3, to which the sample is introduced by pipe 18. Analysis of the resultant ion population may still take place in the region between G3 and G4, which may receive inert gas, and detection of particular species in the region between G4 and A. In another three-grid embodigrouped in accordance with their ment the ion-molecule reaction region between G2 and G3 may be eliminated, as by the elimination of grid G2 and the insertion of the sample into the region between C and G1 (inlet 18 not being used). As indicated above, the sample may be inserted into this region separately from the reactant gas or the sample may contain the reactant gas. Primary ions formed in the region C-Gl will react with the trace gas molecules in this region to form secondary ions, and at a predetermined time grid G1 will open and pass a group of ions to the region Gl-G3. By opening grid G3 at a predetermined time after the opening of grid G1 a portion of the group of ions in the region 61-63 will be passed to the region G3-G4, and by opening grid G4 at a predetermined time delayed with respect to the opening of G3 a selected species of ions will be passed to the detection region G4-A. The time of opening of grid G4 may be varied to scan the drift time spectrum between G3 and G4 and the time of opening of grid G3 may similarly be varied. Non-reactant gases may be introduced into either or both of the successive velocity analysis regions between G1 and G3 and G3 and G4 if desired. In a further three-grid variation the ion-molecule reaction region may be combined with the secondary ion velocity analysis region, as by eliminating G3. A particular species of primary ion passed by G2 after primary ion velocity analysis in region Gl-GZ reacts with sample ions introduced by inlet 18 in region G2-G4, and selected ions are passed by G4 for detection. If desired inert gas may be inserted in region G1-G2.

The apparatus and methods of the invention may be employed to separate, detect, and measure many different types of materials, principally organic materials in gas or water or in solids which may be rendered gaseous. Trace materials in gaseous samples may be measured with a sensitivity of one part in ten billion with almost instantaneous (0.05 second) response. Resolution of 20 to 100 for simple mixtures and specific materials is typical. The apparatus may operate at ordinary atmospheric pressures over a wide range of temperatures from room temperature to 500 F or higher. Sampling may be continuous but need not be. Typical applications are petroleum and chemical industry process control, pollution monitoring, medical applications such as human gaseous effluents and laboratory analysis, agricultural monitoring of pesticides and insecticides, and military uses.

lf theapparatus is operated hot, samples will not adhere to the walls. By selecting the voltage gradient between the electrodes, that is, the principal electrodes and the various grids, the drift time within predetermined regions of the cell may be selected. Thus, the time provided for reactions within the ionmolecule reaction region may be adjusted accordingly. Moreover, several ion-molecule reaction regions may be provided. For example, the primary ions produced by the original reactant gas in region C-Gl may be passed to a first ionmolecule reaction region Gl-G2 for reaction with another gas, and the secondary ions thus produced may be passed to still another region G2-G3 for reaction with the trace gas molecules. A sequence of ion-molecule reactions can be used to form a desired reactant gas which is finally reacted with the trace material. Ion velocity analysis and detection may take place as before. Multiple gases may be inserted in various regions of the drift cell in addition to the region C-Gl. For example, both inert and sample gases may be inserted in ionmolecule reaction regions for concentration control.

While preferred embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that changes can be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims.

The invention claimed is:

1. Apparatus for detecting a substance, in a gaseous sample, which comprises an envelope having a pair of spaced principal electrodes therein, ionizing means associated with one of said principal electrodes for producing ions output means connected to the other principal electrode, means for applying a drift field between the said principal electrodes, at least three ion gates arranged in succession between said principal electrodes, and means for opening said ion gates in succession, the length of the mean free path of said ions in said envelope being very much smaller than the dimensions of said envelope.

2. Apparatus as set forth in claim 1, there being four of said ion gates arranged in succession.

3. Apparatus as set forth in claim 2, said electrodes and said gates being spaced to define sequential regions along said envelope, said apparatus further comprising means for supplying different gases to at least two of said regions substantially simultaneously.

4. Apparatus as set forth in claim 1, said electrodes and said ion gates being spaced to define a first region between a first ion gate and said one electrode, a second region between a second ion gate and said first ion gate, a third region between a third ion gate and said second gate, and a fourth region between said other electrode and said third ion gate, said apparatus further comprising means for introducing difi'erent gases to at least two of said regions substantially simultaneously.

5. Apparatus as set forth in claim 4, and in which said means for introducing one of said gases comprises a source,connected to said first region, of gas which produces primary ions under the influence of said ionizing means and said means for introducing the other of said gases comprises a source of an inert gas.

6. Apparatus as set forth in claim 4, and in which said means for introducing one of said gases comprises a source,connected to said first region, of gas which produces primary ions under the influence of said ionizing means and said means for introducing the other of said gases comprises a source of a substance which produces secondary ions by reaction with said primary ions.

7. Apparatus as set forth in claim 6, further comprising means for introducing into another region of said envelope a gas which is inert to said ions.

8. Apparatus as set forth in claim 4, further comprising means for exhausting gas from said regions.

9. Apparatus as set forth in claim 8, in which said means for introducing and exhausting gases at said regions comprises means for producing a gas flow transverse to the direction between said principal electrodes.

10. Apparatus as set forth in claim 1, and in which each of said ion gates comprises a dual grid having two sets of substantially coplanar grid elements and means for normally maintaining adjacent elements of the grid at equal and opposite potential relative to a predetermined reference potential, said means for opening said gates comprising means for driving adjacent elements of said grids to the same potential momentarily. I

11. Apparatus as set forth in claim 1, in which said apparatus has means for introducing an ion-forming gas therein in the vicinity of said one electrode.

12. A method of detecting a gaseous substance, which comprises forrning primary ions, reacting said primary ions at a first region with molecules of the substance to be detected to form secondary ions, applying a drift field to said secondary ions to cause them to drift in said field, gating a group of said secondary ions to a second region in said field, gating a portion of said group to a third region in said field, and gating a species of said portion to a fourth region in said field and detecting said species.

13. A method of detecting a gaseous substance, which comprises forming primary ions at a first region, applying a drift field to said primary ions, gating a group of said primary ions to a second region in said drift field, gating a portion of said group of primary ions to a third region in said field, reacting said primary ions with molecules of said substance at said third region to form secondary ions, gating a group of said secondary ions to a fourth region in said field, gating a portion of said group to a fifth region in said field,and detecting said portion.

14. A method as set forth in claim 13, in which the ions gated to the respective regions are sorted in accordance with their velocity in said field.

15. method as set forth in claim 13, in which a gas inert to said primary ions is introduced into said second region.

, 16. A method as set forth in claim 13, in which a gas inert to said secondary ions is introduced into said fourth region.

17. A method as set forth in claim 13, in which said substance comprises a gaseous sample introduced into said third region.

18. A method as set forth in claim 13, wherein the length of the mean free path of said ions at said regions is maintained very much smaller than the dimensions of said regions.

19. A method of detecting a substance, which comprises forming primary ions, sorting said primary ions in accordance with-their velocity in a drift field, reacting a portion of the sorted primary ions with molecules of said substance to form secondary ions, sorting said secondary ions in accordance with the velocity of said secondary ions in a drift field, and detecting a portion of the sorted secondary ions.

' 20. A method as set forth in claim 19, in which the sorting of at least said primary ions or said secondary ions takes place in a gas inert to such ions.

. 21. A method as set forth in claim 19, wherein the forming, sorting, and reacting steps recited are performed at regions maintained at a pressure such that the'length of the mean free path of said ions at said regions is very much smaller than the dimensions of said regions.

22. A method in accordance with claim 19, in which the said forming of primary ions comprises the utilization of a continuous radioactive source, and in which the recited forming, sorting, and reacting steps are performed substantially at atmospheric pressure.

23. A method of detectinga substance, which comprises forming ions of said substance, sorting said ions at a first re- 8 gion in accordance with their velocity in a drift field, sorting at a second region in accordance with their velocity in'a'drifi field a group of ions sorted at the first region, sorting at a third region in accordance with their velocity in a drift field a portion of said group of ions sorted at said second region, and sorting at a fourth region in accordance with their velocity a species of said portion sorted at said third region.

24. A method as set forth in claim 23, wherein the length of the mean free path of said ions at said regions is maintained very much smaller than the dimensions of said regions.

25. A method of detecting a substance, which comprises forming primary ions at a first region of a drift field, gating said ions to a second region of said field, reacting said ions with molecules of said substance at said second region to form secondary ions, and detecting at least a portion of said secondary ions.

26. A method in accordance with claim 25, wherein said secondary ions are separated in accordance with their velocity in said field before detection.

27. A method as set forth in claim 25, wherein the length of the mean free path of said ions at said regions is maintained very much smaller than the dimensions of said regions.

28. A method of detecting a substance, which comprises forming primary ions, gating said rimary ions to a first region of a drift field, reacting said primary ions with molecules at said first region to produce secondary ions, gating said secon-' dary ions to a second region of said drift field, reacting said secondary ions at said second region with molecules of said substance to produce further ions, and producing an output signal in response to at least a portion of said further ions. 

2. Apparatus as set forth in claim 1, there being four of said ion gates arranged in succession.
 3. Apparatus as set forth in claim 2, said electrodes and said gates being spaced to define sequential regions along said envelope, said apparatus further comprising means for supplying different gases to at least two of said regions substantially simultaneously.
 4. Apparatus as set forth in claim 1, said electrodes and said ion gates being spaced to define a first region between a first ion gate and said one electrode, a second region between a second ion gate and said first ion gate, a third region between a third ion gate and said second gate, and a fourth region between said other electrode and said third ion gate, said apparatus further comprising means for introducing different gases to at least two of said regions substantially simultaneously.
 5. Apparatus as set forth in claim 4, and in which said means for introducing one of said gases comprises a source,connected to said first region, of gas which produces primary ions under the influence of said ionizing means and said means for introduciNg the other of said gases comprises a source of an inert gas.
 6. Apparatus as set forth in claim 4, and in which said means for introducing one of said gases comprises a source,connected to said first region, of gas which produces primary ions under the influence of said ionizing means and said means for introducing the other of said gases comprises a source of a substance which produces secondary ions by reaction with said primary ions.
 7. Apparatus as set forth in claim 6, further comprising means for introducing into another region of said envelope a gas which is inert to said ions.
 8. Apparatus as set forth in claim 4, further comprising means for exhausting gas from said regions.
 9. Apparatus as set forth in claim 8, in which said means for introducing and exhausting gases at said regions comprises means for producing a gas flow transverse to the direction between said principal electrodes.
 10. Apparatus as set forth in claim 1, and in which each of said ion gates comprises a dual grid having two sets of substantially coplanar grid elements and means for normally maintaining adjacent elements of the grid at equal and opposite potential relative to a predetermined reference potential, said means for opening said gates comprising means for driving adjacent elements of said grids to the same potential momentarily.
 11. Apparatus as set forth in claim 1, in which said apparatus has means for introducing an ion-forming gas therein in the vicinity of said one electrode.
 12. A method of detecting a gaseous substance, which comprises forming primary ions, reacting said primary ions at a first region with molecules of the substance to be detected to form secondary ions, applying a drift field to said secondary ions to cause them to drift in said field, gating a group of said secondary ions to a second region in said field, gating a portion of said group to a third region in said field, and gating a species of said portion to a fourth region in said field and detecting said species.
 13. A method of detecting a gaseous substance, which comprises forming primary ions at a first region, applying a drift field to said primary ions, gating a group of said primary ions to a second region in said drift field, gating a portion of said group of primary ions to a third region in said field, reacting said primary ions with molecules of said substance at said third region to form secondary ions, gating a group of said secondary ions to a fourth region in said field, gating a portion of said group to a fifth region in said field,and detecting said portion.
 14. A method as set forth in claim 13, in which the ions gated to the respective regions are sorted in accordance with their velocity in said field.
 15. A method as set forth in claim 13, in which a gas inert to said primary ions is introduced into said second region.
 16. A method as set forth in claim 13, in which a gas inert to said secondary ions is introduced into said fourth region.
 17. A method as set forth in claim 13, in which said substance comprises a gaseous sample introduced into said third region.
 18. A method as set forth in claim 13, wherein the length of the mean free path of said ions at said regions is maintained very much smaller than the dimensions of said regions.
 19. A method of detecting a substance, which comprises forming primary ions, sorting said primary ions in accordance with their velocity in a drift field, reacting a portion of the sorted primary ions with molecules of said substance to form secondary ions, sorting said secondary ions in accordance with the velocity of said secondary ions in a drift field, and detecting a portion of the sorted secondary ions.
 20. A method as set forth in claim 19, in which the sorting of at least said primary ions or said secondary ions takes place in a gas inert to such ions.
 21. A method as set forth in claim 19, wherein the forming, sorting, and reacting steps recited are performed at regions maintained at a pressure such that the length of the mean free path of said ions at said regions is very much smaller than the dimensions of said regions.
 22. A method in accordance with claim 19, in which the said forming of primary ions comprises the utilization of a continuous radioactive source, and in which the recited forming, sorting, and reacting steps are performed substantially at atmospheric pressure.
 23. A method of detecting a substance, which comprises forming ions of said substance, sorting said ions at a first region in accordance with their velocity in a drift field, sorting at a second region in accordance with their velocity in a drift field a group of ions sorted at the first region, sorting at a third region in accordance with their velocity in a drift field a portion of said group of ions sorted at said second region, and sorting at a fourth region in accordance with their velocity a species of said portion sorted at said third region.
 24. A method as set forth in claim 23, wherein the length of the mean free path of said ions at said regions is maintained very much smaller than the dimensions of said regions.
 25. A method of detecting a substance, which comprises forming primary ions at a first region of a drift field, gating said ions to a second region of said field, reacting said ions with molecules of said substance at said second region to form secondary ions, and detecting at least a portion of said secondary ions.
 26. A method in accordance with claim 25, wherein said secondary ions are separated in accordance with their velocity in said field before detection.
 27. A method as set forth in claim 25, wherein the length of the mean free path of said ions at said regions is maintained very much smaller than the dimensions of said regions.
 28. A method of detecting a substance, which comprises forming primary ions, gating said primary ions to a first region of a drift field, reacting said primary ions with molecules at said first region to produce secondary ions, gating said secondary ions to a second region of said drift field, reacting said secondary ions at said second region with molecules of said substance to produce further ions, and producing an output signal in response to at least a portion of said further ions. 